Let's start with motivation - why the hell would anyone sponsor an F1 team? Exposure. Advertising. You want to get your name out there. RBR and Renault have had a contentious relationship for a while. RBR won a load of championships on Renault engines, and never really acknowledged Renault's role in the matter. They picked a competing car company, Infiniti, as their title sponsor.
Once the new engine regs came out, Renault failed to match Merc for performance. That's bad, and a big failing on their part. Most of the existing Renault-engined teams jumped ship, and the rest are scraping the bottom of the grid. Bad publicity.
In order to start finishing in the top of the grid instead of a lap down, Renault really needs a true works team. That will give them a truly integrated power plant and chassis, and it will let them get their name out a little more publicly. A car in Renault yellow would be good exposure.
Ideally they'd like a team which is already using Renault engines, and they'd like a team which is on sound financial footing. Manor is definitely looking for help, but they're teetering on complete financial meltdown. Buying them also buys their debts. I'm unsure if Force India would consider a sale, but they have their own financial issues, plus they use the wrong engines.
What it boils down to - the only reasonable teams to wear the Renault livery are RBR or STR. RBR just had a max exodus of engineering talent, plus their team principal is notoriously difficult to work with. On top of all that, there's some serious bad blood between RBR management and Renault over performance issues. If Renault came in, they'd have to do a full Ferrari style house-cleaning. I don't know if Redbull proper would stand for that.
Pretty much process of elimination, STR is the only remaining team which is worth buying for Renault. The team has it's own management in place, avoids the problems with RBR. They seem to have some talented designers, a couple of solid drivers, and they're not hideously in debt. They don't have a competing sponsorship deal with another car company. Making the Renault sponsorship more prominent wouldn't seriously impact Redbull proper's visibility in the sport, and would be less likely to cause them to pull back money. And with Renault engineering help during the design phase, they may well start performing better than Redbull. Especially if a Renault deal were to come with some money to hire more aero and chassis talent.